The present invention relates to engraved identification cards and in particular to engraved identification cards with a color changeable layer to provide contrast between two or more layers on an engraved card.
Engraved identification cards are known and can be engraved utilizing any one of a number of electronic engraving systems such as those disclosed in Noda, et al., Pat. No. 3,950,608 or Wada, et al., Pat. No. 4,052,739 or any other similar engraving system. Utilizing an electronic engraving system, an image can be engraved in an identification card blank by making a multiplicity of scores through one opaque colored plastic layer of the card blank into a second, different colored opaque plastic layer of the card blank. By varying the depth of the engraved scorings, the width of the regions between the scores will be varied to generate light and dark regions which make up the desired image. A complete description of such identification cards and the method of making them is disclosed in Oka, et al., Pat. No. 3,897,964 and Oka, et al, Pat. No. 3,930,924.
Heretofore, to provide a useful identification card, it was necessary to initially provide a card blank where an opaque base of one color was overlaid with an opaque layer of a contrasting color so that light impinging on the engraved surface of the card would be variably reflected back to the eye of the observer according to the location and amount of top layer removed to form an observable image.
The present invention comprises a novel identification card and method for making the same by disposing a color-changeable solution (1) over the entire surface of a card blank prior to engraving, (2) in the engraved grooves of a pre-engraved card, or (3) on the non-engraved surface regions of a pre-engraved card to form a solid color changeable layer, and thereafter applying heat for a period of time to effect a change in the color of the color-changeable layer to provide contrast between the color-changeable layer and the base layer. Initially the color-changeable layer will preferably be the same color as the base layer. To assure such a color coordination, it is possible to pigment the base layer to match the color of the color-changeable layer. One preferred pigmenting compound used which is heat sensitive to cause a color change is sodium 12-molybdosilicate. That this pigment changes color in the presence of heat is disclosed in Crone, et al., U.S. Pat. No. 3,078,182 which describes the use of sodium 12-molybdosilicate with vinyl copolymers in a pressure-sensitive adhesive formulation which is used for autoclave sterilization indication.
However, the formulation disclosed by Crone, et al., results in a color change only after exposure to heat in excess of about 350.degree. F. which could cause a polyvinyl chloride (PVC) base in accordance with the invention to be damaged. Further, there is no disclosure of diffusion bonding or crosslink bonding to a PVC card or the use of the color changing pigmenting agent to enable "developing" of an engraved card.